1. Field of the Invention
This invention relates generally to lubricating oil compositions and specifically to such an oil composition which has high oxidative-stability at elevated temperatures.
2. Prior Art
It is widely accepted that a 2,6-di-tert-butyl-p-cresol, a compound known as DBPC, is highly effective as an oxidation inhibitor for lubricating oils such as typically turbine oil. However, DBPC per se of a relatively low molecular weight is vulnerable to evaporation upon exposure to elevated temperature over prolonged length of time resulting in a loss of oxidative-stability of a turbine oil.
Japanese Laid-Open Patent Publication No. 60-156644 discloses the use of sterically hindered hydroxyphenylcarboxylic acid ester as a stabilizing agent for synthetic organic polymers, animal and vegetable oils, hydrocarbons, lubricants and the like. A specific example of this stabilizing agent is disclosed to be (3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid ester. While this compound is much more resistant to diminution by evaporation compared to the 2,6-di-tert-butyl-p-cresol, it is less effective in terms of oxidation-inhibition.